Add .idea/vcs.xml to .gitignore

[BenHenning]

This file doesn't actually need to be shared across the team, and it's been the cause of some confusion.

[BenHenning]

This needs to be verified by me locally before I can send it for review. My Android Studio is currently occupied, though, so it may be a little bit before I get a chance to check it.

[BenHenning]

This seems to work correctly: when I open Android Studio on this branch it silently replaces vcs.xml and that doesn't show up in the staging area since it's being ignored now. I don't use Git within Android Studio, but the new file doesn't seem particularly special, so I suspect that this should work for everyone.

I suggest we submit this as-is and see if anyone actually reports issues.

[BenHenning]

FYI @Victor-Titan

[anandwana001]

LGTM, good to go with this, Thanks @BenHenning

[BenHenning]

Thanks @anandwana001! Merging this now.

[MohamedMedhat1998]

@BenHenning After merging with the latest develop, I lost my .idea/vcs.xml. I can get it from any project but I suggest that you send an email about that to Oppia-android-dev so that people are aware of this PR.

[BenHenning]

As mentioned on the Hangouts chat, you should be able to reopen Android Studio after taking updating to the latest develop & it will recreate .idea/vcs.xml (at least, that's what I observed).